Method and apparatus for molding article in plurality of colors

ABSTRACT

A method and apparatus for molding plastic articles in two colors, particularly, the bodies of magnetic tape cassettes, in which a strong bonding force is provided between the two portions without having to form a recess and projection on the joining edges of the two portions. A slide core, which is moved in and out of a primary mold cavity between the mold of the two portions of the article, is provided with a tubular passage having an opening within the primary mold cavity along edge portions of the slide core. The slide core is fully inserted into the primary mold cavity to mold the first portion, then partially withdrawn to form a seconary mold cavity in which the second portion is subsequently molded. After molding the first portion, an adhesive is injected through the tubular passage along the edges of the first portion which will join corresponding edges of the second portion to thereby firmly join the two portions.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for molding aplastic article in a plurality of colors, and more particularly to amethod and apparatus for molding an article in a plurality of colors insuch a manner that the strength of a joint between primary and secondarymolded pieces made of plastics of respective two different colors isenhanced.

Products made of injection-molded plastics are widely used, and variousmethods for injection molding have been cassette.

A magnetic tape cassette generally employs a pair of hubs on which amagnetic tape is wound rotatably supported in the body of the cassette.The cassette body is composed of upper and lower half portionsmanufactured by the injection molding of a plastic material such as ABSresin. Each of the upper and lower half portions has a transparent ortranslucent window through which the state of winding of the magnetictape or the like can be observed from the outside. The window portion,which is made from a different material from that of the main part ofeach of the upper and the lower portions, is conventionally joined tothe main part by an adhesive, ultrasonic fuse-bonding or the like.Therefore, the window portion must be manufactured separately from themain parts of the upper and the lower portions and then later joinedthereto. This results in less than desirable productivity and a highcost of production.

For this reason, a so-called two-color molding method for manufacturinga magnetic tape cassette, which does not require a separate step ofjoining the window portion, has been often used recently. However, thebonding force between the mutually joined surfaces of the two types ofplastic used in the two-color molding process to constitute the windowportion and the half portions of the body of the magnetic tape cassettestrongly depends on the temperature and injection pressure of thelater-injected plastic material, and the strength of the joint of themutually joined surfaces is generally less than desirable. This is aproblem particularly if the magnetic tape cassette is of a type having arelatively small body thickness and is made of two different types ofplastic materials which are not soluble in each other. The problem isvery serious because it affects the quality and service life of thewhole cassette.

In an attempt to overcome this problem, a construction has been proposedin which the stepped joining surfaces of the upper and the lower halfportions are provided to increase the mutual contact areas of thejoining surfaces and to thereby enhance the strength of the joint.However, the bonding force is still generally insufficient.

In another proposed magnetic tape cassette, one side of the joinedsurface of the upper and lower half portions of the body of the cassetteor the window portion is provided with a projection located at themiddle of the joined surface in the direction of thickness of thecassette body, and the other side of the joined surface is provided witha recess in which the projection is fitted so that the two molded piecesare engaged with each other. Such a proposal is disclosed in JapaneseUnexamined Published Utility Model Application No. 60083/85.

However, this approach is still disadvantageous in that the size of theprojection is limited, and hence the bonding force is stillinsufficient. More specifically, the molding process employs a slidecore which slides in the direction of thickness of the body of thecassette and which has a recess extending perpendicularly to thatdirection so as to create the projection on the joined surface inmolding each of the upper and lower half portions of the cassette body.This slide core must be moved after injection molding of the halfportion. When the slide core is moved, if the molded projection islarge, the recess in the slide core will greatly strain the projectionmolded therein when the slide core is moved. That is, if the size of theprojection is made large enough to sufficiently increase the bondingforce between the window portion and the body portion, the projectionwill undergo strain or deformation due to the movement of the slidecore, thus again decreasing the strength of the joined surfaces.

SUMMARY OF THE INVENTION

The present invention was made in order to solve the above-describedproblems.

Accordingly, it is an object of the present invention to provide amethod and apparatus for molding plastic articles in a plurality ofcolors in which the bonding force between the mutually joined surfacesof two different types of plastics sufficiently high enough to providethe joint with sufficient strength, even if the surfaces are notprovided with an engaging projection and recess.

In the method provided in accordance with the present invention, a firstmolten plastic material is injected into a cavity in which a slide corehas previously been inserted, the slide core is thereafter moved todefine another cavity, and the molten second plastic material is theninjected into the latter cavity. In accordance with the presentinvention, a tubular passage, which can be connected to the outside ofthe molding die, is provided in the slide core, an adhesive is suppliedfrom outside the slide core through the tubular passage and allowed toflow out between the slide core and the molded piece formed from thefirst plastic material to thereby apply the adhesive to the boundarysurface of the piece, and the molten second plastic material isthereafter injected into the latter cavity defined by moving the slidecore.

Further, a molding apparatus of the invention for molding an articlehaving first and second portions formed of different plastic materialsincludes a fixed mold member, a movable mold member defining a moldcavity with the fixed mold member, at least one slide core on themovable mold member which is slidable in and out of the mold cavity andhaving at least one passage formed therein having at least one openingat a lateral edge of the slide core inside the mold cavity, and meansfor supplying an adhesive through the passage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a magnetic tape cassette which may bemolded using a plural-color molding method of a preferred embodiment ofthe present invention; and

FIGS. 2 and 3 are sectional views of major parts of mold members forplural-color molding the cassette of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is hereafter describedin detail with reference to the attached drawings.

FIG. 1 shows an audio-type magnetic tape cassette 10 of a generallyconventional overall configuration which can be invention. The body 2 ofthe cassette 10 is composed of upper and lower half portions 3a and 3bhaving reel shaft insertion holes 12 into which are loosely fittedrotatable hubs 4 on which a magnetic tape 5 is wound. The front of thecassette body 2 has a central opening 13 into which a recording/playbackhead can be inserted, small openings 14 located in symmetric positionsat the right and left of the central opening 13, and openings 15 locatedin symmetric positions at the right and left of the openings 14. Thecassette body 2 also has capstan insertion holes 16 and positioningholes 17.

Each of the upper and the lower half portions 3a and 3b of the cassettebody 2 has two windows 11a and 11b. The cassette body 2 is made of anopaque plastic except for the windows 11a and 11b, which are made of atransparent plastic so that the magnetic tape 5 in the cassette body canbe seen through the windows. The windows are molded from a transparentplastic using a two-color molding process described with reference toFIGS. 2 and 3.

FIGS. 2 and 3 are vertical sectional views of the mold members 20 and 21taken along a line I--I in FIG. 1. FIG. 2 shows the mold members in afirst step of molding each of the upper and lower half portions of thecassette body 2. FIG. 3 shows the mold members 20 and 21 in a secondstep of molding the windows. One of the mold members is fixed moldmember 20 and the other is a movable mold member 21 separable from thefixed mold member 20. The movable mold member 21 is fitted with slidecores 40a and 40b. which can be moved in the direction of thickness ofthe cassette body 2 (in the direction of arrows A in FIGS. 2 and 3) byhydraulic cylinders installed under the movable mold member 21 (but notshown in the drawings). The slide cores 40a and 40b have tubularpassages 41a and 41b formed therein which are connected to the outsideof the movable mold member 21. The tubular passages 41a and 41bbifurcate near the top of the slide cores 40a, 40b and open in the sideof the cores 40a, 40b near the top thereof. An adhesive S is suppliedfrom outside the movable mold member 21 through the tubular passages 41aand 41b to the edge surfaces of each of the upper and the lower halfportions 3a, 3b, which are molded from a first, opaque plastic material,and the adjoining edge surfaces of each of the window portions 11a, 11b,which are molded from a second, transparent plastic material. The edgeportions of the slide cores 40a and 40b are tapered upward to formspaces, when the cores 40a, 40b are moved downward,. into which it iseasy for the adhesive S to flow from the cores 40a, 40b.

Rigid pipes 50 are connected to the tubular passages 41a and 41b, Themovable mold member 21 has openings 52 in which the pipes 50 are allowedto be moved up and down. The pipes 50 are connected to flexible pipes 51opposite the tubular passages 41a and 41b.

The main part of each of the upper and the lower half portions 3a and3b, that is, the entire half portions except the windows 11a and 11b,are molded in a first step. To mold the main part, the slide cores 40aand 40b are first moved upward to fill secondary mold cavities 33corresponding to the windows 11a and 11b, and the first molten plasticmaterial is then injected into a primary mold cavity 31 through aboundary gate 32. The main part of the half portion 3a or 3b is thusmolded.

After the first plastic injected into the primary mold cavity 31 hassolidified,. a prescribed small quantity of the adhesive S is injectedunder pressure between the edge portions of the molded first plasticmaterial and each of the slide cores 40a and 40b from outside themovable mold member 21 by a hydraulic cylinder (not shown in thedrawings) to thus apply the adhesives to the surfaces of the main partof the half portions 3a, 3b which are to be joined to the windows 11aand 11b.

Before injecting the adhesive S to the surfaces of the main part, theslide cores 40a and 40b may be slightly moved down to define clearancesbetween the main part of the half portions 3a, 3b and the cores 40a,40b. In that case, the adhesive S can be easily applied to the main partof the half portions 3a, 3b even if the pressure of the adhesive to beinjected is relatively low.

The adhesive S is preferably an ultraviolet-setting resin such as anunsaturated polyester resin, a urethane or an acrylate resin. Theadhesive S can be injected into the clearances immediately after thecores 40a and 40b are slightly moved down. If desired,. the timing ofinjection can be set by a timer (not shown in the drawings) which isstarted simultaneously with the movement of the cores 40a and 40b.

After the adhesive S is applied to the main part of the half portions 3aand 3b, the slide cores 40a and 40b are moved down in the direction ofthe thickness of the cassette body 2, as indicated by arrows A in FIG.3, so that the tops 42 of the cores 40a, 40b are located in positionscoincident with that of the inside surface of the cassette body. As aresult,. the secondary mold cavities 33 corresponding to the windows 11aand 11b to be molded are opened. The second plastic material is theninjected into the cavities 33 through internal gates 23a and 23b to moldthe windows 11a and 11b.

When the fixed mold member 20 and the movable mold member 21 arethereafter separated from each other to permit extraction of the moldedhalf portion 3a or 3b of the cassette body 2, ultraviolet rays areirradiated upon the adhesive S through the windows 11a and 11b to hardenthe adhesive S to thus firmly join the windows 11a, 11b to the main partof the molded half portion 3a, 3b. The irradiation can be performed at adesired time point.

Since the windows 11a and 11b are joined to the main part of the halfportions 3a, 3b not only by the heat and injection pressure of thesecond plastic but also by the adhesive S, the bonding force between thejoined surfaces of the main part of the half portions 3a, 3b and thewindows 11a,11b is increased to make the strength of the joint of thejoint high enough to withstand external forces acting on the surfaces inthe direction of thickness of the cassette body 2. Even if the first andthe second plastics are not soluble in each other,. the windows 11a and11 are joined to the main part of the half portion 3a, 3b firmly enoughto withstand normally encountered external forces.

Although each of the tubular passages 41a and 41b has two outlet portsin the side of the corresponding slide core 40a, 40b near the topthereof in the above-described embodiment, the present invention is notconfined thereto, but may be otherwise embodied so that each of thetubular passages has one outlet port or three or more than three outletports. Also, the upper edge portions of the slide cores 40a and 40b maybe straight rather than tapered.

Moreover, although in the above-described embodiment the adhesive S canbe injected into the clearances between the main part of the upper andlower half portion 3a or 3b and the slide cores 40a and 40b immediatelyafter the cores 40a, 40b are slightly moved down to define theclearances, the adhesives can be also be injected between the main partof the half portions 3a, 3b and the cores 40a, 40b simultaneously withthe downward movement of the cores 40a, 40b or immediately before thedownward movement. Further, although it has been described that theadhesive S is injected between the main part of the half portions 3a, 3band the slide cores 40a and 40b using a hydraulic cylinder, the adhesiveS may be injected without using a hydraulic device.

Still further, the adhesive S may be other than anultraviolet-ray-setting resin, for example, it may be a conventionalresin such as a thermosetting resin.

Although the above-described embodiment relates to a molding methodemploying two plastic materials in two colors, the present invention isnot confined thereto, but may be embodied as a method of molding in morethan two colors.

In a plural-color molding method provided in accordance with the presentinvention. The primary molded part of each half portion of the body of amagnetic tape cassette is molded, an adhesive is thereafter supplied tothe surface of the primary molded part which is to be joined to thesurface of the secondary molded part of the half portion, and thesecondary molded part is then molded. As a result, the bonding forcebetween the joined surfaces of the primary and the secondary moldedparts depends not only on temperature and injection pressure, but alsoon the action of the adhesive so that the strength of the joint is madehigh enough to withstand external forces acting in the direction ofthickness of the body of the cassette.

With the inventive method, the mutually joined surfaces need not beprovided with an engaging projection and recess. Thus, the joinedsurfaces do not suffer strain or deformation due to the formation of theprojection and the recess as in the prior art. Furthermore, even if theplastic materials used to mold the first and the second molded parts arenot soluble in one another, the strength of the joint is still madesufficiently high that the parts are prevented from being separated fromeach other by normally encountered external forces.

What is claimed is:
 1. A method for molding an article having first andsecond portions formed of different plastic materials, comprising thesteps of:inserting a slide core into a primary mold cavity; injecting afirst plastic material into said primary mold cavity to mold said firstportion of said article; allowing said first plastic material injectedinto said primary cavity to harden; at least partially withdrawing saidslide core from said primary mold cavity while said first plasticmaterial remains firmly in place in said primary mold cavity to form aspace for receiving an adhesive; injecting a small amount of an adhesivethrough at least one passage in said slide core along an edge of saidfirst portion to be joined to an abutting edge of said second portion toadhere said edge of said first portion to said abutting edge of saidsecond portion; further withdrawing said slide core from said primarymold cavity; and injecting a second plastic material into a secondarymold cavity left by withdrawal of said slide core to mold said secondportion of said article, wherein only said adhesive is injected throughsaid slide core.
 2. The molding method of claim 1, wherein said firstand second plastic materials are not soluble in one another.
 3. Themolding method of claim 1, wherein said first plastic material is opaqueand said second plastic material is transparent, said second portionbeing a window portion of said article.
 4. The molding method of claim1, wherein said passage is a tubular passage having a bifurcated endportion opening to opposite sides of said slide core.
 5. The moldingmethod of claim 1, wherein upper end portions of said slide core aretapered to form recesses for receiving said adhesive.
 6. The moldingmethod of claim 1, wherein said step of injecting said adhesive isperformed after said step of at least partially withdrawing said slidecore.
 7. The molding method of claim 1, wherein said step of injectingsaid adhesive is performed simultaneously with said step of at leastpartially withdrawing said slide core.
 8. The molding method of claim 1,wherein said adhesive is an ultraviolet-setting resin selected from thegroup consisting of unsaturated polyester resin, urethane and acrylateresin, epoxy and acrylate resin, and polyester and acrylate resin. 9.The molding method of claim 1, wherein said adhesive is a thermosettingresin.